Using Upnp for Port Forwarding - Wan Interface

I would like to know if it is possible to use Upnp to open/forward a port to another router connected to the Wan interface?

I am interested because my setup is a router behind a router. I would like to open port 3389 for rdp connections. I can set up manual port forwarding rules in the second router, but I cannot in the first. Is there a command I can issue from my second router to se up a port forwarding rule using Upnp.

There are no built-in command-line tools I'm aware of to induce a UPnP forward on an upstream device (or a local device for that matter). The protocol is annoying/complex.

If you are able to install Entware (assuming your second router has a USB port and you have a small USB stick you can use for extra space), there is a package called miniupnpc (note the c, not d! d is the daemon and comes with many TomatoUSB builds, c is the client) which you can install. The client/program to run is called upnpc. It should be able to do what you need.